Strictly speaking, we do not know the category of the element that has been relativised. Perhaps it could just as well be an NP. However, temporal adverbials are usually expressed as PPs in examples that do not involve relativisation, as in the example below:

Example 2

a.

Ik gean om dy tiid fuort

I go at that time away

I leave at that time

b.

*Ik gean dy tiid fuort

I go that time away

I leave at that time

It is clear from the contrast above that the temporal adverbial corresponds to a PP.